Apple Business Manager: Add existing Apple IDs to the account

Hey there everyone!

I'd need your help because I'm trying to set-up Apple Business Manager for my organization (Small Company) and I'm facing a problem:

We already have 8 devices each with its dedicated (previously created) Apple ID and I'd need to create new account on Apple Business Manager for these Apple IDs.

Thing is though, I can't seem to create any account since I get the error: "This managed Apple ID is taken".

I mean we created a bunch of Apple IDs prior to Apple Business Manager but it's not being managed by anyone in particular and I know for a fact that these accounts are not currently set-up in another Apple Business Manager account.

So my question is this: How can I add existing Apple IDs to Apple Business Manager ?

Thanks in advance!